Comparison of Fibrillar Adhesives (to
glass)

(please email if you have updates to this data)
NM= not measured

There has been much confusion in the press recently about the
performance of various ``gecko-like'' products. One point of confusion
is the comparison of nano-scale contacts, typically done with an
atomic force microscope, to large patch areas. This is an
apples-to-oranges comparison, as sophisticated hierarchical structures
are used in the gecko to get adhesion over large areas, and any material in a nanoscopic
contact will have a huge pressure. (Pressure is force divided by area,
so as area goes to zero, the pressure tends to infinity.) Another
source of confusion is the nature of what makes a sticky material. Soft
rubbers and the soft materials used in pressure sensitive adhesives
such as Scotch tape are naturally sticky
because they can make conformal contacts with hard surfaces, as well as
having appropriate viscosity properties. Soft sticky materials can make
strong attachments, and by making fibers and stalks of soft
material, patches with interesting adhesive properties can be
made.
A second point is the attachment and detachment of a gecko adhesive.
A gecko adhesive does not require being pressed into a surface- the fibers
engage by being dragged parllel to the surface with minimal normal force.
A gecko adhesive exhibits ``frictional adhesion'' where the fibers push
off the surface if there is not a force parallel to the surface, giving
automatic release.

Notes:
1) Shear adhesion coefficient is the ratio of pull-off shear stress to
normal preload stress. A high adhesion coefficient means only a light contact
will be needed to engage the adhesive.
2) Frictional adhesion is the key property where the normal pulloff force
goes to zero when shear force is removed. In this way, the gecko can remove
its foot with zero detachment force.

Macroscopic Patches

Common tests for measuring gecko adhesive performance include the peel
test, the normal pulloff test, and shear adhesion strength.

Hard Polymer Arrays

Product

Material

Modulus

90 degree
Peel
(N/m)

Pull-off
(N)

Shear
(N)

Area
sq. mm.

Normal
preload
(N/sq.cm)

adhesion
coeff.

Effective
Modulus
(kPa)

Natural Gecko, Autumn et alNature 2000

beta keratin

2 GPa

~0

1

10

100

nil (<0.01)
(shear engaged)

8-16

100

Gecko Lamellar prep, Lee et al JRS Interface 2008

beta keratin

1.5 GPa

NM

~0

0.27

0.5

<0.05

>5

100

Davies et al.Int. J. Adh. Adh. 2008

polyimide
on SEM tape

2 GPa

NM

0.53

~0

196

0.25

2

NM

GeimNature Materials 2003

polyimide
on Scotch tape

3 GPa?

NM

3

NM

100

50

0.06

3000
(w/o buckling)

Lee et al.JRS Interface 2008

polypropylene

1 GPa

< 0.1 N/cm

~0

4 N

240

0.05

~30 (shear)

~200

Lee et al.APL 2008

polyproylene

1 GPa

<0.1
N/cm

~0

9N

200

<0.1

~45
(shear)

~200

Schubert et al.J. Adhesion Sci. Tech 2007

polypropylene

1 GPa

< 0.1 N/cm

~0

1 N

1000

0.05

~2 (shear)

~200

Kustandi et al.Adv. Funct. Mat. 2007

parylene

2.8 GPa

NM

0.7

NM

100

1

0.7

NM

Jeong et al.Coll. Surf. 2008

PMMA

2.8 GPa

NM

~0

3

100

<1

3
(shear)

NM

Carbon Nanotube Arrays

Product

Material

Modulus

90 degree
Peel
(N/m)

Pull-off
(N)

Shear
(N)

Area
sq. mm.

Normal
preload
(N/sq.cm)

adhesion
coeff.

Effective
Modulus
(kPa)

Ge et al PNAS 2007

carbon nanotube
on Scotch tape

1000 GPa

2-5

0.8

1-6

16

25-50

<.1

~200

Qu and Dai Advanced Materials 2007

carbon nanotube
on Si

1000 GPa

NM

~5

~2.5

16

125

<.2

NM

Qu et al. Science Oct. 2008

carbon nanotube
on Si

1000 GPa

NM

~3

~16

16

125

<0.1

NM

Maeno and Nakayama APL 2009

multiwall carbon nanotube on
polypropylene

1000 GPa

NM

NM

45

100

50

0.9

NM

Zhao et al J. Vac. Sci. Tech. 2006

carbon nanotube
on silicon

1000 GPa

0.08

0.5

0.6

8

>500

<.01

~200

Soft Polymer Fiber Arrays

Product

Material

Modulus

90 degree
Peel
(N/m)

Pull-off
(N)

Shear
(N)

Area
sq. mm.

Normal
preload
(N/sq.cm)

adhesion
coeff.

Effective
Modulus
(kPa)

Sameoto and MenonJMM 2009

PDMS
(Sylgard 184)

1.8 MPa

NM

9mN

NM

1

2

5

~400

Yoon et al.Nano Today 2009

polyurthane acrylate + Pt

19.8 MPa

NM

NM

31

100

0.3

100

NM

Jeong et al.PNAS
2009

polyurethane acrylate

19.8 MPa

NM

15

78

300

0.3

70

~26

Kim and SittiApplied Physics Letters
2006

polyurethane

3 MPa

0.07

0.07

NM

0.4

12

1.5

~300

Murphy, Aksak, Sitti, Small 2008

polyurethane

3 MPa

NM

5

10

100

NM

NM

~20

Gorb et al JRS Interface, 2006

PVS

3 MPa

~1

0.4

NM

7

2

2.9

~300

Parness et al.JRSI 2009

PDMS

1.75 MPa

NM

0.5

1.7

100

0.25

2.1

~50(?)

Santos et al.J. Adh. Sci. Tech. 2007

polyurethane

0.3 MPa

NM

1

~1

390

0.25

4-13

~100(?)

Sitti and Fearing
IEEE ICRA 2003

PDMS

0.5 MPa

NM

0.003

NM

100

0.025

0.1

~100

Davies et al.Int. J. Adh. Adh. 2008

PDMS

0.6 MPa

NM

1.2 N

NM

123

0.25

40

~100

Ge et alPNAS 2007

Scotch brand tape

0.1 MPa?

NM

NM

6

16

NM

NM

<100

Shan et alIEEE Nano/Micro 2006

PDMS

2.5 MPa

NM

2.01

NM

100

NM

NM

~240

Spherical Probe Tests

Load, drag, pull test

Spherical indentation test Load then Pull off

Product

Material

Modulus

Probe Radius
(R)

Preload

Load-Pull
Normal Force

and

F/R (N/m)

Load-Drag-Pull
Normal Force

and

F/R (N/m)

Adhesion Coefficient

Schubert et alJRS Interface 2008

polypropylene

1 GPa

5 cm

2 mN

< 0.05 mN
(< 0.001 N/m)

0.8 mN
(0.016 N/m)

0.4

Northen and TurnerCurrent Apl. Phys 2006

organorods

~1 GPa

0.3 cm

12 mN

0.45 mN
(0.15 N/m)

NM

0.04

Kim and Sitti APL 2006

polyurethane

3 MPa

0.3 cm

22 mN

65 mN
(20 N/m)

NM

3

Murphy et al.App. Mat. and Int. 2009

polyurethane

3 MPa

1.2 cm

256 mN

600 mN
(50 N/m)

NM

2.3

Murphy et alJAST 2007

polyurethane

3 MPa

0.3 cm

4 mN

8 mN
(3 N/m)

6 mN
(2 N/m)

1.5

Greiner, del Campo, ArztLangmuir 2007

silicone rubber

2.6 MPa

0.25 cm

2 mN

1.1 mN
(0.5 N/m)

NM

0.5

Greiner, Arzt, del CampoAdv. Materials 2009

silicone rubber

1.9 MPa

0.25 cm

0.5 mN

0.07 mN
(0.03 N/m)

NM

0.14

Nanoscopic Patches

These
are measurements of very small numbers of contacts over a very
small area. The results at the nanoscale can not be directly
extrapolated to performance of macroscopic patches. In many cases an
atomic force microscope is used to make contact with one to several
fibers.
Notes:
(1) For natural gecko, we are assuming ``frictional adhesion''
mode, where a shear force is necessary for the pulloff force to be
non-zero.
(See Fig. 4 of Autumn et al Nature 2000). The shear and normal is
estimated by assuming 100 to 1000 spatulae are in contact. Each spatula
is assumed to be 200x200 nm.